Getting pregnant again less than 18 months after you last gave birth increases the risk that you'll go into labor well before your due date. An analysis of almost half a million women found that more than one in five who conceived less than a year after their previous pregnancy gave birth before 37 weeks.

Your baby is growing by leaps and bounds, you miss that pregnant glow, and you're dreaming of giving your son or daughter a sibling to play with. Is it time to add to your family again? Not so fast, say researchers who have just crunched the latest numbers on the link between short intervals between pregnancies on preterm birth risk. If it's been less than a year and a half since your last pregnancy, it might be safer to wait.

"The shortest intervals between pregnancies have the largest magnitude of effect on preterm birth risk," says study author Emily DeFranco, D.O., of the University of Cincinnati.

DeFranco and her colleagues looked at all birth records in the state of Ohio from 2006 to 2011 in which a woman gave birth to a single baby and had a prior pregnancy on record; 454,716 cases fit the criteria. They analyzed the interval between pregnancies, the length of each pregnancy, as well as other known risk factors for pre-term births, including race, socioeconomics, age, body mass index and diabetes.

Overall, about 2 percent of women included in the study had become pregnant less than a year after their last pregnancy, and almost 11 percent had pregnancies spaced a year to 18 months apart; the remaining 87 percent had more than 18 months between pregnancies. But women who were black, had lower levels of education or smoked were even more likely than average to have a short interval between pregnancies.

The statistics revealed that shorter breaks between babies meant shorter gestations, on average. Of those women with the least time between pregnancies — less than a year — more than 20 percent gave birth before 37 weeks gestation and more than half before 39 weeks. Only ten percent of women who waited a year to 18 months to get pregnant, on the other hand, gave birth earlier than 37 weeks, and less than eight percent of women who waited more than 18 months had babies that early.

The reason that shorter intervals between pregnancies boosts the risk of preterm births isn't fully understood, DeFranco says. "There are metabolic stressors and perhaps nutritional depletion," she says. A woman's body may not be able to build up sufficient levels of some vitamins and nutrients when she is still recovering from a recent pregnancy. Other theories suggest that inflammation that occurs in the uterus during pregnancy needs time to subside.

What this means to you. There are many factors to consider when deciding how to space your children, and the risk of preterm birth is just one. Doctors generally recommend waiting about a year after you give birth to start trying for another baby, but the risks or benefits may be quite different from woman to woman, so longer or shorter might be best.

"It really depends on what the individual person's risk of preterm birth is," says DeFranco. "If someone has had several full term babies in the past and is overall at low risk, then I would say wait at least six months. But if someone is at high risk of preterm birth and just had an early preterm birth, that would be the type of woman I would say should do anything she can to really minimize her risk and perhaps wait 18 months or longer."

And on the flip side, waiting longer than five years between pregnancies can put you at high risk of a low-birth weight baby or other complications. The sweet spot in sibling spacing (likely somewhere between one and five years) will be whatever gap you decide — armed with all the latest research on health drawbacks and benefits, sibling rivalry and parental sanity — is best for you family.

From the What to Expect editorial team and Heidi Murkoff, author of What to Expect the First Year. Health information on this site is based on peer-reviewed medical journals and highly respected health organizations and institutions including ACOG (American College of Obstetricians and Gynecologists), CDC (Centers for Disease Control and Prevention) and AAP (American Academy of Pediatrics), as well as the What to Expect books by Heidi Murkoff.